1. Field of the Invention
This invention relates to a method of processing a digital audio signal. In addition, this invention relates to an apparatus for processing a digital audio signal. Furthermore, this invention relates to a computer program for processing a digital audio signal.
2. Description of the Related Art
According to the CD standards, an analog audio signal is converted into a digital audio signal by sampling and quantizing processes with a sampling frequency fs of 44.1 kHz and a quantization bit number of 16. Generally, a sampling frequency and a quantization bit number are basic factors which determine the quality of audio contents represented by a resultant digital audio signal. A higher sampling frequency and a greater quantization bit number provide a better audio quality. In the case where a sampling frequency fs is 44.1 kHz, signal components having frequencies of 22.05 kHz or higher are cut off.
To improve audio quality, the DVD video standards and the DVD audio standards use a quantization bit number of 24 and a sampling frequency fs of 48 kHz, 96 kHz, or 192 kHz.
It is known to re-quantize and up-sample a digital audio signal of the CD standards into a digital audio signal of the DVD video standards or the DVD audio standards which corresponds to a sampling frequency of 48 kHz and a quantization bit number of 24. In this case, the audio waveform represented by the DVD digital audio signal is basically identical with that represented by the original CD digital audio signal, and the DVD digital audio signal also lacks signal components representing audio frequencies of 22.05 kHz or higher. Therefore, in this case, an improved audio quality can not be provided.
A brief explanation will now be given of conceivable systems which are not prior art against this invention. The conceivable systems are designed to add high-audio-frequency signal components to an original digital audio signal to get a modified digital audio signal representative of audio contents with enhanced presence in auditory sensation. In the conceivable systems, the addition of high-audio-frequency signal components to the original digital audio signal is independent of the audio frequency represented by the original digital audio signal. Therefore, it is difficult to add high-audio-frequency signal components to only components of the original digital audio signal which represent audio frequencies in a desired band or a limited band.